It is well known that there are a large number of man-made thermoplastic polymers and resins which can be melt-extruded as monofilaments for the making of chopped fibers, or as continuous filaments. Many of these can be extruded as multifilaments or fine denier filaments. Fibers and filaments of large denier (generally greater than 15 denier) can usually be prepared from any thermoplastic polymer or resin which has a melt strength sufficient to withstand the tensile forces (gravitational or supergravitational) acting on the weakest portion of the extrudate.
The following references contain general information about the preparation and characteristics of high-denier monofilaments, low-denier multifilaments and staple fibers of olefin polymers: Encyclopedia of Polymer Science and Technology (1968), Vol. 9, pp 403-440, published by Interscience, New York; Kirk-Othmer Encyclopedia of Chemical Technology, (1981) third edition, Vol. 16, pp. 357-385, John Wiley & Sons, New York; Plastics World, June 1984, page 113; Man-Made Fiber and Textile Dictionary, published by Celanese Corporation; Man-Made Fibres by R. W. Moncrieff, John Wiley & Sons, New York; Fundamentals of Fibre Formation by Andrizej Ziabicki, John Wiley & Sons, New York. One specific technique for making microporous hollow fibers from various polyolefins is disclosed in U.S. Pat. No. 4,670,341. The hollow fibers of U.S. Pat. No. '341 are said to be formed by melt spinning a polyolefin/filler/plasticizer composition, followed by an extraction procedure, thus forming the microporous hollow fibers. The extraction procedure is chosen such that the polyolefin and the filler are insoluble.
Olefin/unsaturated carboxylic acid copolymers (especially ethylene/acrylic acid) in the form of pellets may be digested with an alcohol/caustic mixture causing the pellets to spontaneously convert into colloidal particles (U.S. Pat. No. 3,798,194) or the pellets may be digested in an amine and agitated to obtain small, non-colloidal, particles and fibers (U.S. Pat. No. 3,790,521). Such olefin/carboxylic acid copolymers are said to be hot-drawn as strands, the strands chopped into pellets, and the pellets digested with an alkali to obtain, upon shearing, small fibers (U.S. Pat. No. 3,801,551).
Ethylene polymerized with unsaturated organic carboxylic acids (ECA), e.g., acrylic acid (EAA), to form copolymers is taught, e.g., in U.S. Pat. No. 2,391,218; U.S. Pat. No. 3,520,861 and U.S. Pat. No. 4,351,931. Copolymers of ethylene and such acids can be made by grafting the acid onto polyethylene, by batch or continuous polymerization of mixtures of monomers of ethylene and the acid, by polymerization of mixtures of monomers of ethylene and the acid in a tubular reactor, and hydrolysis of copolymers of ethylene/alkyl acrylates which converts the ester groups to carboxylic acid groups. Also, block copolymers can be made whereby chain segments of polyacrylic acid and chain segments of polyethylene form long polymer chains. Any of these known ethylene/acid copolymers are suitable for use in the present invention, so long as they are of sufficient molecular weight to be formed into solid particles, fibers or filaments. Thus, the purview of the present disclosure includes, inter alia, ethylenic polymers containing a plurality of carboxylic acid groups in their molecular structure.
In addition to the ethylenic/acid (ECA) copolymers and terpolymers, other olefinic copolymers and terpolymers are within the purview of the present invention so long as the polymer is one which has reactive or reactable groups among the polymer chain as pendant side-groups which can be substantially reacted with a reagent, but where the polymer backbone substantially retains its molecular integrity.
It has now been discovered that there are unexpected characteristics resulting from a change in the known process of forming fibers of certain polymers, such as ethylene/unsaturated carboxylic acid (ECA) copolymers, said known process being that of digesting extruded strands of the polymer in a reagent which substantially reacts with side-groups pendant from the polymer molecule and then subjecting the so-treated polymer to shearing forces to cause fibrillation or particulation of the polymer strands. The said change in the process involves, as the principal distinguishing feature, the orientation (i.e. "stretching") of the polymer at a temperature below that at which stress-relaxation of the stretched polymer molecules is substantially encountered; this may be referred to as "cold-stretching" or "crystalline orientation".
It is customary, in some production processes, for polymers leaving a polymerization vessel to be melt-extruded through a die as strands which are chopped into pellets and cooled in a water bath. U.S. Pat. No. 3,801,551 discloses that such pellets may then be digested in alkaline material and then fibrillated using shearing forces. Since the strands are cut into pellets before the strands have cooled to the point at which crystallization occurs, then stress relaxation of the polymer molecules permits intertwining of the molecules. When such pellets are treated in accordance with U.S. Pat. No. 3,801,551 to prepare fibers, the fibers are relatively short and have very little porosity, if any. Polymers produced as small particles may also be melted, extruded as strands, and chopped into pellets before the intended end-use.
It has been found that if the extruded strands of polymer are allowed to cool to an extent, and for a time, sufficient to allow an appreciable amount of crystallization and are then cold-stretched (oriented), the crystallized molecules become substantially untwined and become substantially aligned in parallel relationship with the direction of orientation. This also draws the strands to narrower dimensions. Since the polymer, when oriented, is cold enough for the molecules to be crystallized, and not warm enough to allow stress-relaxation of the molecules, then the molecules remain dimensionally stable after the orientation is completed. When these oriented strands are treated with a reagent which reacts with the side-groups, (e.g. 0.5N NaOH) and subjected to shearing or crushing, the strands undergo fibrillation into fibers which are extensively porous. Most of the pores are small enough to be considered micropores. These micropores permeate the length and breadth of the fibers. Even if the oriented strands are chopped into pellets before being treated with alkali and subjected to shearing or crushing, the so-formed fibers are extensively porous and are longer than fibers prepared from pellets of the same dimensions treated in accordance with U.S. Pat. No. 3,801,551.
This same phenomenon is observed when cold-oriented films or strips are treated with alkali and sheared or crushed into fibers, and to a greater degree than when using films or strips which are stretched at high temperatures where stress relaxation of the polymer molecules is possible.